The field of this invention is that of removing blockages in remote subsea pipeline, typically from subsea gas well installations.
Hydrates are a porous solid which is formed primarily of water with a mixture of gases. It is effectively similar to ice. There is a tendency for hydrates to form in the pipelines departing from a subsea gas well, especially on well startup.
The temperature of seawater at depths will often approach 32° F., with the temperature in non-flowing pipelines being the same. When a subsea pipeline valve is opened, the gas expansion can cause substantial additional cooling. In these cold and high pressure conditions, hydrates of the gas and water can quickly form.
Frequently when the hydrate forms, it forms a blockage, the blockage will be somewhat porous. At that time, a high pressure will exist on the upstream side and a lower pressure will exist on the downstream side of the blockaged. This means that additional gas will move thru the hydrate and expand and therefore cool as it does. This means that not only can the expansion of this gas keep the formed hydrate cool, but can literally continue to grow additional hydrate blockage.
It is difficult to tell where the hydrates are actually located in deepwater pipelines, especially when the pipelines are buried.
Hydrates formed like this can last for weeks or months, with a substantial loss of gas flow and therefore revenue to the owner of the pipelines and subsea wells.
Paraffins can form blockages in pipelines by building up on the inner diameter of the cold pipelines as relatively warm oil circulates out of an oil well and cools as it flows down a subsea pipeline. As the layer of paraffin builds up on the subsea pipeline inner diameter, the inner diameter of the paraffin becomes smaller and smaller. Ultimately a pigging device intended to clean the paraffin will cause the paraffin to separate from the inner wall of the pipeline and become a plug. In some cases the paraffin will release from the subsea pipeline inner diameter without a pig and cause a blockage. In either case, if the pressure in the pipeline is enough to move the plug along the pipeline, it will continue to collect additional paraffin as it moves until the length of the blockage cannot be moved by the available pressure.
Some attempts have been made to enter the end of the pipeline with a somewhat flexible string of coiled tubing to get to the blockage and wash it out. This is an expensive operation, and in some cases the blockage can be 10 to 20 miles away.
Removal by use of coiled tubings can be further complicated if the pipeline has bends in it, making passage of the coiled tubing difficult if not impossible.